Inevitably as a series string of cells goes through its service life, a variety of events and conditions conspire to ensure that during discharge, one cell discharges fully sooner than its neighbors, and that during charge, one cell charges fully sooner than its neighbors. This prompts investigators to try to devise ways to balance the charge among the cells in the string. Experience shows, however, that it is not easy to balance the state of charge when several electrochemical cells are in series. For example if one wishes to selectively charge particular cells (for example to “top up” a particular cell that needs topping-up), the charging module for any particular cell needs to have isolation relative to any charging modules for other cells. The charging modules for the cells likewise need to have isolation relative to any external energy source being drawn upon for charging purposes.
But it is not enough merely to find a way to provide isolation mechanisms for the various charging modules. It is also necessary to find a way for each module to be individually controlled as to the current being applied by that module to its respective cell. The control mechanisms might be “local” to the respective cell or might be centralized. If centralized, then the control mechanisms must also be electrically isolated as needed.
A reader hoping to gain valuable background in the area of cell balancing and charge redistribution will find it helpful to review the following patent documents:                U.S. Pat. No. 6,518,725 B2 Marten issued Feb. 11, 2003        U.S. Pat. No. 6,511,764 Marten issued Jan. 28, 2003        WO 2008-137764 A1 published Nov. 13, 2008        U.S. Pat. No. 7,936,150 B2 to Milios issued May 3, 2011        WO 2012-042401 published on Apr. 5, 2012        WO 2012-056417 published on May 3, 2012        
Investigators have proposed any of a wide variety of approaches for such balancing and charging. A patent of possible interest is US 2010-0295509 A1 to Moussaoui et al. published Nov. 25, 2010. A review of past proposed approaches reveals many drawbacks to various approaches. For example many approaches using inductive coupling require “snubbers”, circuits to fight and to absorb transients that develop when current to an inductor is cut off. Snubbers for high-voltage circuits are particularly tricky to design. In the absence of a snubber, or in the absence of a snubber that is good enough to do the job, such a transient can lead to failure of the controlling device such as a switch.
Some approaches are costly in terms of the number or physical bulk of switches, inductors, or capacitors employed (per cell) to bring about the balancing or charging. Some approaches are disappointing in terms of the energy losses suffered during the balancing process. Some approaches only achieve charging based upon an external energy input but cannot redistribute charge between cells in a string. Some approaches only serve to discharge particular cells, throwing away energy merely to ensure that no cell performs better than the weakest cell in the strong.
It would be helpful if a family of approaches could be devised that would permit selective charging and balancing of cells in a string, using a minimum of expensive or bulky components per cell, with maximum efficiency and minimal operational losses.